FIG. 1 illustrates a prior art electric power generating system (EPGS) of the type manufactured by the assignee of the present invention for use in airframes. The EPGS 7 is comprised of a plurality of generating channels 37. Each channel comprises a generator unit such as an integrated drive generator 9 coupled to a power take-off (not illustrated) from an airframe propulsion engine. The output of the IDG 9 is connectable by a generator control breaker (GCB) 31 to a load distribution bus 23 which is connectable by a bus tie breaker (BTB) 29 to a tie bus 35. Each IDG 9 is conventional and is comprised of a constant speed transmission and a permanent magnet generator which generates alternating current which is rectified and applied to a wound field exciter which produces alternating current which is rectified and applied to the rotor of a three phase alternator. The number of IDGs 9 included in the EPGS 7 varies directly with the number of engines on the airframe and typically is between 2 and 4. The rotor of the three phase alternator is driven by the constant speed transmission (included within the IDG 9) which converts a variable speed power take-off from the airframe propulsion engine into a constant speed shaft drive which rotates the rotor of the three phase alternator at a velocity for producing three phase 400 Hz electrical power. Each IDG 9 has an associated generator control unit (GCU) 11 which may contain a programmed microprocessor or other means for implementing various conventional control and protection functions as well as functions which are described below which are part of the present invention.
In addition to the main engine driven IDGs 9, an auxiliary power unit driven generator unit (AGEN) 13 is often included as an integral pan of the EPGS 7. The AGEN 13 is connectable by an auxiliary power breaker (APB) 39 to the tie bus 35 to allow the AGEN 13 to power the main channel load busses 23 via the BTBs 29 during IDG fault or loss of engine conditions, or while on the ground without main engines running. As with the IDGs 9, the AGEN 13 has an associated generator control unit (AGCU) 15 which also may contain a programmed microprocessor or other means for implementing various conventional control and protection functions. Often the GCU 11 and the AGCU 15 are identical units, differing only in the control algorithms executed by the microprocessor. Also included is a connection 17 to allow external power (EXT PWR) to be connected to the tie bus 35 to supply the main channel's load distribution busses 23 through the BTBs 29 while on the ground.
The generator control unit 11 is conventional. The GCU 11 contains a generator control relay (GCR) (not shown) which controls the connection of electric power generated by the permanent magnet generator to the wound field exciter via line 19 which upon disconnection disables the generator unit from generating electric power. In the prior art EPGS 7 the GCR is a latching relay which opens in the event of a GCU failure or a protective trip, de-energizing the IDG 9 thereby. In this way the generating channel with the associated failed GCU 9 reverts to a safe configuration with the IDG 9 off line.
The protection logic executed within the GCU 11 utilizes a generator current transformer housed within the IDG 9 to monitor the current generated by the IDG 9. The current information is transmitted to the GCU's protection logic via line 21 where it is compared to the current information as monitored by a line current transformer located at the load distribution bus 23 and transmitted via line 25. If the current generated by the IDG 9 does not equal that being delivered to the load distribution bus 23, a differential current bus fault (a single or multiple phase to ground or phase to phase short circuit) exists. In response to the sensed fault, the protection logic trips open the BTB 29 via BTB control line 27 and continues to monitor for the existence of the fault. If the fault persists, the protection logic trips open the GCB 31 via GCB control line 33 and also de-energizes the IDG 9 by allowing the GCR to open. In similar fashion an overcurrent condition, a single or multiple phase to ground or phase to phase short circuit fault downstream of the line current transformer (on the load distribution bus itself), will result in the isolation of the load distribution bus 23.
For generator faults such as over or under voltage generation as sensed on line 41 at the point of regulation 43, the protection logic trips open the GCB 31 to disconnect the IDG 9 from the load distribution bus 23, allows the GCR to open to de-energize the IDG 9 and closes the BTB 29 to allow an alternate source to power the loads via the tie bus 35.
In addition to the protection logic functions described above, protection against GCU microprocessor faults and power interruptions is included within the GCU 11 to allow the channel 37 to fail in a safe configuration, hereinafter fail-safe protection. Traditionally, a nonvolatile latching device set by the main protection is used by the fail-safe protection to determine the appropriate control action in the event of a GCU failure or power interruption. By using this information, the fail-safe protection can properly configure the channel 37 during the fault or interruption, as well as reconfigure it after the GCU 11 is reset or power is restored. If the fail-safe logic were to close the BTB 29 to allow an alternate source of electrical energy to power the load distribution bus 23 without first determining if a bus fault had existed, for example as a result of a differential current fault being present on the load distribution bus 23, the alternate source of electrical energy could be connected to the faulted load distribution bus 23, resulting in the loss of, or potentially damage to, the alternate source. For a two engine aircraft an error of this type could have very serious consequences resulting in the loss of primary power to the entire aircraft. Two problems associated with using this separate nonvolatile latching device to ensure non-propagation of a bus fault are that it 1) increases the cost and 2) decreases the reliability of the GCLI 11 based on the increased parts required.
The present invention is directed to overcoming one or more of the above problems through the use of a latching multiple pole GCR, improved fail-safe logic, and the deletion of the nonvolatile latching device.